Yagi antennas are used in high frequency applications such as satellite radio transmission. There presently exists a population of 4 billion people that are generally dissatisfied and underserved by the poor sound quality of short-wave or terrestrial radio broadcast systems. This population is primarily located in Africa, Central and South America, and Asia. FIG. 1 shows an overview of a satellite broadcast system 10 comprising various broadcast stations 2 for transmitting multiple audio signals, for example, to a satellite 4, which in turn transmits these signals to the receivers 9. The satellite broadcast system 10 is particularly useful for providing high-quality broadcast programming to users in Africa, Central and South America, and Asia. The present invention relates to a low-cost antenna that can be mounted on a portable radio receiver 9 for reception of satellite radio transmissions. This invention is particularly useful for the reception of satellite signals where a receiver antenna gain on the order of 9 dbi together with a noise figure on the order of 1 dB are required due to the low power flux density available at the receiver location.
Yagi antennas generally consist of three types of elements: reflector, radiator, and directors. The radiator (e.g., a folded dipole) is an active element that receives the power concentrated by the parasitic elements. The reflector is a parasitic element with an inductive quality. The directors are also parasitic elements but with a capacitive quality. Yagi antenna systems use the parasitic elements in combination with active elements to control the direction and width of the beam. The Yagi antenna optimizes gain by using specific director lengths and spacing between the directors and the driven element (e.g., the radiator).
In addition, the Yagi antenna typically employs a balun (e.g., a half wavelength coaxial line) to achieve a 180 degree phase shift of the signal. Specifically, as seen in FIG. 2, a coaxial cable 32 is physically connected to the driven element (e.g., folded dipole) 40. The inner sheath 38 is connected to one side of the folded dipole 40 and the feed cable 34, and the opposite inner sheath 39 is connected to the opposite side of the dipole. The outer sheath 36 is connected to ground. As the signal travels around the inner sheath from 39 to 38 it becomes 180 degrees phase shifted from the original signal. This cable and dipole arrangement is cumbersome and prevents an antenna arrangement from being constructed on a simple printed circuit board. A need exists for a more compact means to drive the components of a Yagi antenna.
A compactly designed Yagi antenna is disclosed in U.S. Pat. No. 5,612,706. However, this antenna merely reduces the distance between two rods and is not well suited for radio receiver portability. It is more convenient to have a Yagi antenna that can be folded for transportation. Further, it would be advantageous to have a less costly implementation than the one disclosed in U.S. Pat. No. 5,612,706.
Removal of the balun is described in U.S. Pat. No. 5,898,410. A log periodic dipole array antenna system achieves impedance matching by adjusting the distance between a focusing element and one of several dipoles or driven elements. The antenna system therefore has plural active elements and, correspondingly, impedance matching requirements for each of these elements. A need exists for a low-cost antenna having a simple active element impedance matching design.
A performance limitation of the Yagi antenna is the signal loss caused by cables and connectors between the antenna feed point and the low noise amplifier input stage. There is currently a requirement to match the antenna feed point to a standard impedance (such as 50 ohms) which can be accommodated by off-the-shelf connectors and cables, and then again match the impedance to the low noise amplifier input stage. This sequential impedance matching requirement incurs line and connector losses, which in turn detrimentally affect the performance of the Yagi antenna.
As shown in FIG. 3, some patch and Yagi antenna systems 10 use dual circular polarization outlets which can be costly due to the type and number of components. For example, the system shown requires two outlets, that is, a right-hand circular polarization outlet 18 and a left-hand circular polarization outlet 16, two low-noise amplifier (LNA) input stages 24 and 26, an electronic polarization switch 14, and at least two housing mounts 12 and 13.
Manufacturing costs are also a contributing factor to the expense of the receivers 9. It is known in the art to use coaxial cables 20 and 22 to connect the LNA input stages 24 and 26 to the antenna outlets 18 and 16 to achieve impedance matching. However, as mentioned in U.S. Pat. No. 4,518,968, balanced low impedance feeders have been recommended, but have not often been adopted in practice. This is because such feeders, when engineered for dipole and Yagi-Uda array matching impedances, are dimensionally awkward to manufacture and install. Further, since the folded dipole and the director elements are separate from the low noise amplifier (LNA), two fabrication procedures are needed, thereby increasing the likelihood of problems due to manufacturing tolerances. Thus, a need exists for a low cost Yagi antenna design that is easily mass-produced with a low error tolerance.
It is known, for example, from U.S. Pat. No. 5,272,485, to use antennas embedded in substrates in microwave frequency applications where a feedpoint and via are used as an input to a low noise amplifier, thereby obtaining optimum impedance matching. However, these diagonally-fed electric microstrip dipole antennas are patch antennas that are constructed on at least two layers of a dielectric substrate. These types of patch antennas cannot be designed for high gain without using an array of patches, thereby incurring a negative effect on complexity and size.
Accordingly, a need exists for a more simple means of impedance matching of a Yagi antenna with only one driven element. A need also exists for an active antenna system that is low cost and readily mass-produced while providing reasonably high gain, directivity and noise performance. A foldable design is desirable to keep the antenna compact for travel.